Electric cars and hybrid cars have been commercialized, in which a drive-motor and a storage battery for driving the drive-motor are mounted to those cars. The hybrid car, in particular, fits modern life and draws market attention.
FIG. 8 shows a driving system of a hybrid car available in the market. An engine, a drive-generator and an air-conditioning compressor are mounted in the car. The engine drives both of the compressor and drive-wheels. A control system includes a control unit (ECU), a DC/DC converter and an inverter. A storage battery of 36V is mounted for driving the drive-generator, and a storage battery of 12V is mounted for driving the control system and a starter. Both of the storage batteries are of the same size and mounted in a trunk (boot). The generator generates electricity and charges both the batteries while the car runs, i.e., prepares for further use, namely, discharging. When the 36V storage battery drives the generator, a transmission of the engine is set at a neutral position.
Meanwhile, cars employed storage batteries of 6V at first, which were replaced by 12V batteries, and now are going to be replaced by 42V batteries that guarantee 36V as discussed above. The battery drives not only a generator but also an air-conditioning compressor, and further use of storage batteries can be expected.
The storage battery, as discussed above, is charged while the engine is driven, so that it can be used as long as possible. However, self-heating of the battery shortens its expected service-life. The self-heating is generated by a chemical reaction in charging and discharging. When a temperature rises, dilute sulfuric acid gas runs away, which erodes the electrodes, so that the service life of the battery is shortened. The battery sometimes discharges several kilowatts at 36V, namely, 200–250A. Thus, if a temperature of the battery rises by more than 10° C. from an operating temperature ranging from 50 to 60° C., the service life is reduced by half.
In order to overcome such a problem, Japanese Patent Application Non-Examined Publication No. H09-289701 discloses that a lithium-ion battery employs a method of reducing discharging-power step by step in two steps.
In the case of a lead-acid battery, charge/discharge is controlled so that off-charge and off-discharge can be done in two steps within a temperature-range from 60 to 70° C. for overcoming the problem discussed above. Charge/discharge is desirably thus controlled in an early stage; otherwise, a thermal runaway occurs and a temperature of the battery rises instantaneously to as high as 80–90° C.
In the case of a nickel metal hydride battery, Japanese Patent Application Non-Examined Publication No. H10-270095 discloses an air-cooling apparatus including a fan, for instance. Various methods of providing an air-cooling path are proposed in order to cool the battery effectively.
However, those discharge restricting methods discussed above force the users to use the battery only for a short period due to the temperature rise of the battery, and the battery needs a long time for recovery. Thus a hybrid car employing one of those methods cannot fully enjoy the advantages of the hybrid. In other words, the car runs with gasoline rather than with the batteries. Thus it is desired to increase the ratio of battery-driving vs. gasoline-driving.
In the case of air-cooling used with the nickel metal hydride battery, the battery is still protectively controlled, and the cooling effect needs improvement for increasing the ratio of battery-driving.
The lead-acid battery functions even at a temperature ranging from as low as −5 to −30° C., although its performance lowers by 20–30%. On the other hand, the nickel metal hydride battery and the lithium-ion battery cannot work properly at a low temperature, thus they need to be warmed up. However, conventional temperature regulating techniques for warming up cannot deal with severe low temperatures in cold areas.